Flashcards in Lecture 9: Introduction to Cardiac Electrocardiography (ECG) Deck (72):
What produces extracellular and body surface electrograms?
Propagation of transmembrane action potential
What is an electrogram?
A recording of electrical potential difference
Shows the difference between the positive pole and negative pole
If positive lead is +1 mV and negative lead is -1mV then you have 1 – (-1) = +2mV
What is an electrode?
A contact with the body
What is a lead?
An arrangement of electrodes configured into positive and negative poles
A lead has a vector orientation
What are the factors that alter the amplitude of the vector of electrical activity?
1. The mass of muscle generating the signal
Example: hypertrophy increases amplitude
2. Conduction velocity
3. Degree of cancellation due to propagation in different directions simultaneous
If the orientation of the lead and the vector of electrical activity are parallel, what do you see on the electrogram?
The highest amplitude possible
If the orientation of the lead and the vector of electrical activity are perpendicular, what do you see on the electrogram?
How many electrodes make up the standard 12 leads of a standard ECG?
How are leads I, II and III placed?
The Three “L” rule
Lead I = 1 L = Right to Left Arm = 0 and 180 degrees
Lead II = 2 L’s = Right Arm to Left Leg = 60 to -120 degrees
Lead III = 3 L’s = Left Arm to Left Leg = 120 to -60 degrees
What are the frontal leads?
Leads I, II and III
How are leads aVL, aVR, aVF oriented?
aVL = Left arm (positive pole) = -30 degrees to +150 degrees
aVR = Right arm (positive pole) = -150 degrees to +30 degrees
aVF = Left leg (positive pole) = +90 degrees to -90 degrees
What are the augmented frontal leads?
aVR, aVL, aVF
What does aVR stand for?
Augmented vector right
What are the precordial leads (chest leads)?
V1 – V6
What is the negative pole in the precordial leads?
The central terminal of Wilson
Created by connecting each of the three limb electrodes through 5000 Ohm resistors
It behaves as if it is located in the CENTER of the chest
What is the central terminal of Wilson?
The negative pole for all the precordial leads
Where are the precordial electrodes placed?
V1 = 4th intercostal space just right of the sternum
V2 = 4th intercostal space just left of the sternum
V3 = halfway between V2 and V4
V4 = 5th intercostal space in mid clavicular line
V5 = Lateral to V4 in the anterior anxillary line
V6 = Lateral to V4 in the mid axillary line
What is the ECG appearance of a normal sinus rhythm?
Upward P wave needs to be seen in V1, V2 and aVF
What is the order of the sinus rhythm events?
1. SA node depol
2. Atrial depol
3. Atrial repol
4. AV node depol
5. Bundle of his depol
6. Vent depol
7. Vent. Pleateau
8. Vent. Repol
What forms the P wave?
What forms the QRS complex?
What forms the ST segment?
What forms the T wave?
What sinus rhythm events cannot be seen in a normal sinus ECG?
1. SA node impulse
2. depolarization of the AV node
3. repolarization of the atria
4. depolarization in the His bundle and bundle branches
What is a Q wave?
Part of the QRS complex
An initial downward deflection BEFORE any R wave
Ventricular depol spreading from left to right, specifically left bundle to the septum!
As seen in the frontal leads
What causes the R wave in V1?
When electrical activity goes from left bundle to septum
What is a R wave?
Part of the QRS complex
The first UPWARD reflection
Can be present in the absence of Q
Ventricular depol spreading from right to left
What is a S wave?
Part of the QRS complex
A downward deflection AFTER an R
What is a R’ (R prime)?
A SECOND upright deflection
Follows an S wave
Why is the ST segment at zero voltage baseline?
Because all ventricular muscle cells are at similar plateau voltage so there is no potential difference
What does the vertical scale represent in an ECG?
What does the horizontal scale represent in an ECG?
So Voltage vs. Time
How do the 12 orientations of the ECG related to one another temporally?
They are all being measured at the EXACT SAME TIME
How are the precordial leads classified?
1. septal leads (V1,2)
2. anterior leads (V3,4)
3. lateral leads (V5,6)
How do you systematically interpret ECG?
3. Intervals (PR, QRS, QT)
4. QRS axis
5. Configuration (P wave, QRS, ST segment, T wave)
How do you calculate heart rate?
300/# of interval boxes between R waves
How can you tell a beat is supraventricular or ventricular?
Supraventricular beats (e.g. sinus rhythm) have a P wave before QRS complex
What constitutes a normal sinus rhythm?
Rhythm is regular (R-R interval is consistent)
P waves are UPRIGHT in leads Ieads I, II, III and aVF
PR < 200ms
If HR 60 = sinus tachycardia
What is the significance in the PR interval?
The delay of the AV node before you get to the ventricular contraction
Also shows that atrial depol is aight
What is a normal PR interval?
0.12 – 0.20 s
3 to 5 small boxes
Anything longer than 0.2s means AV nodal conduction is longer than normal
What is the significance of a PR > 0.2 s?
First degree AV block
AV conduction delay
What is the significance of the QRS duration?
How long it take to spread charge across the ventricle
What is normal for QRS duration?
Normal < 0.10s
Normal = both left and right bundle branches rapidly reach all parts of both ventricles
Abnormal > 0.12s
What is the significance of a QRS > 0.12s?
Slow ventricular depol
Not using left and right bundle branches
i. Bundle branch block
ii. Ventricular origin
What is the significance of the QT interval?
Beginning of ventricular depol to end of ventricular repol
What is a normal QT interval?
If it is 2 big boxes, you have a heart rate of 60 beat/min
Smaller than 2 big boxes = faster heart rate
What is the significance of prolonged QT?
Prolonged ventricular APs
What is the normal range for QRS axis?
-30 to +90 degrees
What leads to abnormalities in the QRS axis?
Loss of muscle mass due to infarction
Why is the T wave of positive amplitude?
Because repolarization starts in the direction of the last action depolarized depolarized in the ventricle (so it goes from last to first).
Two opposites cancel out (because repol is opposite direction from depol generally, but repol is going from last fibers of ventricular depol to first fibers rather than first to last)
Epicardium Aps are shorter than Endocardium so that’s why repol goes from last to first
When do you usually see the T wave point the opposite direction of QRS complex?
When there is a wide QRS complex
This is because repolarization goes from first to last in this case
How is the sequence of ventricular depol reflect in QRS configuration?
First thing to get depol in ventricle is interventricular septum (Left bundle branch to septum) will mean Q wave in frontal leads
R wave = activation of Purkinje network towards apex of heart (from endocardium to epicardium)
S wave = charge going from apex to top of ventricle
How is ventricular depol reflected in precordial leads?
R wave = left bundle branch to interventricular septum from V1 to V5
Then Q wave in V6
What does absence of initial septal activation R wave in V1 mean?
What does absence of increasing R wave in V1 – V5 suggest?
How do you determine QRS axis?
Look at lead I and lead aVF
Then take the line perpendicular to lead I and aVF
If lead I and aVF have positive QRS amplitudes, then the wave is somewhere between 0-90 degrees
Find the isoelectric lead (the one that has amplitudes which most closely cancel out to 0) and the QRS axis will be in the direction of the line PERPENDICULAR to the isoelectric lead
What is T wave positive deflection rather than negative deflection, even though it is reflective of repolarization?
It is indicative of the DIFFERENCE between Action potentials of endo and epicardium
Loss of difference = downtick of the T wave instead
What are the key characteristics of PRIMARY AV block?
AV DELAY WITHOUT block PR > 0.2s
What are the key characteristics of SECONDARY AV lock?
Intermittent AV block
Some Ps followed by QRS but some Ps are NOT followed by QRS
What are the key characteristics of tertiary AV block?
COMPLETE heart block between atria and ventricles
Every P wave are not paired with QRS complex
What are the three best leads to look for bundle branch blocks?
What is the difference between bundle branch block and AV block?
BBB modifies duration and morphology of QRS complex but does not prevent ventricular activation
Because left ventricle is bigger, most QRS reflects LV activation
What are the key characteristics of RBBB?
R wave is normal in lead 1, V6
But BROAD S wave in leads 1 and V6 while RVR’ in V1
What does broad S wave in I and V6 + R’ in V1 indicate?
What is the M shaped pattern in V1 known as?
RSR’ = RBBB
What are the key characteristics of LBBB in ECG?
Leads I and V6 = wide R wave (since it takes longer to depol left ventricle)
Lead V1 = wide S wave
What are the standard units used in ECG?
1. Rate (bpm)
2. PR (msec)
3. QRS (msec)
4. QT (msec)
5. QRS Axis (degrees)
Describe the essential ECG features of normal sinus rhythm with normal AV conduction (practice question).
Upward p wave in leads I, II and aVF for supraventricular beat
BPM = 60-100
PR interval = 120-200ms
QRS interval <400ms
Why does the QT interval vary with heart rate? (Practice Question)
Because it is inversely related (faster repol in order to have higher HR)
During ventricular tachycardia would you expect the QRS duration to be less than or greater than 0.12sec and why? (Practice Question)
Less so that the heart can beat faster
Summarize the difference in ECG appearance of complete AV nodal block and LBBB. (Practice Question)
In complete AV nodal block, you see that the p wave and QRS complex is completely disjointed because electrical conductance cant get through the annulus fibrosus
In LBBB, the p waves and QRS complex is synchronized, the only difference being that the R waves in leads I and V6 are very wide and the S wave in V1 is wide